The chronically catheterized fetal sheep is being used as an experimental model to study the function of oxytoxin, AVP, and prostaglandins in the initiation and maintenance of parturition. Initial studies show that fetal and maternal plasma oxytocin concentrations rise late in the first stage of labor and into the second stage. These data demonstrate that the role of oxytocin in parturition is supportive rather than initiatory. Native PGF2 alpha and its metabolite PGF2 alphaM (13,14-dihydro-15-PGF2 alpha) is being measured by radioimmunoassay. These measurements are being conducted in fetal plasma and maternal utero-ovarian vein plasma in normal control pregnancies and in pregnancies where labor is induced by a stepwise administration of cortisol to the fetus. In addition, the effect of administration of PGF2 alpha, AVP, or oxytocin to both ewe and fetus is being studied. The endocrine data is being correlated with a detailed biophysical assessment of uterine contractions. A precise and objective recording technique and data storage system has been developed and modified extensively to quantify uterine activity and associated biophysical events. This is the single most important aspect of this investigation, since never before have the biophysical events been quantitated and so closely correlated with the endocrine sequence of parturition. Quantitative analysis of these correlates is underway.